1. Field of the Invention
The present invention relates to an antenna device for receiving broadcast waves, and particularly to an antenna device for receiving terrestrial digital broadcasts by a portable device used in a state of being worn by a human body.
More specifically, the present invention relates to an antenna device formed by utilizing a cable for transmitting audio signals to earphones, and particularly to an antenna device that has excellent antenna characteristics in both a VHF band and a UHF band.
2. Description of the Related Art
The service of terrestrial digital broadcasting using a terrestrial UHF band (470 MHz to 890 MHz) started in three major wide areas, that is, the Kanto area, the Kinki area, and the Chukyo area on Dec. 1, 2003. Terrestrial digital broadcasting can provide high-definition television programs of high image quality and high sound quality as well as interactive programs. Terrestrial digital broadcasts can be received by a UHF antenna, and can be received and viewed clearly without flicker even by a television installed in a running train or bus, for example. In addition, a service is planned which allows simple moving pictures, data broadcasts, and audio broadcasts to be received and viewed by a portable information terminal or the like.
While terrestrial digital broadcasting is a broadcasting system that performs transmission with a band of six megahertz divided into 13 segments, a one-segment partial reception service, or a so-called “one-seg” service, which distributes video, audio, and data for portable telephones and mobile terminals by only one central segment of the 13 segments, is scheduled to be started on Apr. 1, 2006 (Saturday), and is drawing attention. The one-seg program service provides basically the same contents as programs for ordinary television receivers which programs are distributed by using the 12 segments. Therefore popular programs usually viewed by a user on a television installed in a house can be enjoyed while the user is out. As receiving terminals, various receivers such as portable telephones, car navigation systems, personal computers, dedicated portable televisions and the like are expected to appear.
In addition, terrestrial digital radio broadcasting started on Oct. 10, 2003. One distribution system of terrestrial digital radio broadcasting is a three-segment-based digital broadcasting in which a seventh channel in a current VHF band (188 MHz to 200 MHz) is divided into eight segments and one channel is composed of three of the eight segments. This system enables a wide variety of service of audio, simple picture, and data broadcasts, and enables not only indoor reception but also clear reception by a vehicle-mounted receiver such as a car radio or the like and a portable receiver of a pocket type or the like.
As receiving antenna for relatively small broadcast wave receiving devices, rod antennas are widely used, and telescopic rod antennas, helical antennas and the like are known. When terrestrial digital broadcasting for portable devices spreads, highly sensitive reception performance needs to be secured to deal flexibly with various viewing modes. The present inventors et al. consider that a method of forming a receiving antenna is one important technical problem especially when a portable device includes a multiple tuner for different required frequency bands of FM radio broadcasting using a VHF band and terrestrial digital TV broadcasting using a UHF band, for example.
It is known in the art that an earphone cable is coupled with an antenna circuit and the earphone cable is used as a wire antenna for a portable radio device. An earphone antenna of this type makes it possible to realize a broadcast receiving antenna with high portability by a relatively simple configuration and incorporate an antenna of an inconspicuous design without hampering efforts to meet needs for lighter weight, smaller size, and portability.
For example, an earphone-microphone has been proposed in which when a cord connecting the earphone-microphone and a portable telephone with each other is formed by a parallel two-core cable composed of a pair of an acoustic signal line and a ground line, the ground line is utilized as an antenna, whereby the whole of the earphone cable is used as antenna (see Japanese Patent Laid-open No. 2005-354275, for example).
In addition, an earphone antenna has been proposed which functions as a dipole antenna and obtains high gain over a wide band (see Japanese Patent Laid-open No. 2005-333613, for example). This earphone antenna includes a shield cable having one end connected to a main body of a radio set via a multipolar connector and an earphone cable connected to another end of the shield cable via a connecting block. The earphone cable is formed by two insulation-coated signal lines for supplying audio signals to earphones. The shield cable includes a coaxial line formed by covering a central conductor transmitting a high-frequency signal with an insulator and further covering the insulator with a shield line, insulation-coated signal lines and an insulation-coated ground line for audio signals, and a shield line covering the outside of the coaxial line, the signal lines, and the ground line with an insulator between the shield line and the outside of the coaxial line, the signal lines, and the ground line. The signal lines and the ground line for audio signals are connected to the earphone cable via high-frequency chokes each having a low impedance in a frequency region of the audio signals and having a high impedance in a frequency region of the high-frequency signal, whereby transmission lines for the audio signals are formed. The earphone antenna uses the earphone cable and the shield line covering the shield cable as an aerial, the earphone cable and the shield line being connected to the coaxial line via a balun. The earphone antenna thus has a dipole antenna structure resonating at line length of the aerial. The length of each of the earphone cable and the shield line is adjusted so as to receive 100 MHz in the VHF band.
This earphone antenna resonates at 100 MHz and is able to perform reception at 200 MHz as 1-λ antenna when the characteristic impedance of the coaxial line is adjusted to 75Ω, the length of the shield cable is adjusted to 70 cm, and the length of the earphone cable is adjusted to 50 cm. While it suffices to use harmonic excitation (third harmonics, fifth harmonics, and seventh harmonics) of 100 MHz and 200 MHz in the UHF band, the earphone antenna is basically an antenna element ready for the VHF band. The VHF band and the UHF band are rather distant from each other on a frequency axis, and thus it is difficult to satisfy respective antenna characteristics of the VHF band and the UHF band simultaneously.
Alternatively, a central conductor of a coaxial line within a shield cable is connected to one signal line of an earphone cable, and connected to another signal line of the earphone cable via a capacitor having a high impedance in a frequency region of audio signals and having a low impedance in a frequency region of a high-frequency signal. Signal lines for the audio signals on the shield cable side are respectively connected to two signal lines on the earphone cable side via high-frequency chokes having a low impedance in the frequency region of the audio signals and having a high impedance in the frequency region of the high-frequency signal, whereby transmission lines for the audio signals are formed. A shield line covering the outside of the coaxial line and audio signal lines within the shield cable is connected to a ground. The earphone cable and the shield line are used as an aerial and operate as a sleeve antenna structure resonating at a line length of the aerial. The length of each of the earphone cable and the shield line is adjusted so as to receive 100 MHz in the VHF band (see Japanese Patent Laid-open No. 2005-348252, for example).
This earphone antenna resonates at 100 MHz and is able to perform reception at 200 MHz as 1-λ antenna when the characteristic impedance of the coaxial line is adjusted to 75Ω, the length of the shield cable is adjusted to 70 cm, and the length of the earphone cable is adjusted to 50 cm. While it suffices to use harmonic excitation (third harmonics, fifth harmonics, and seventh harmonics) of 100 MHz and 200 MHz in the UHF band, the earphone antenna is basically an antenna element ready for the VHF band. The VHF band and the UHF band are rather distant from each other on a frequency axis, and thus it is difficult to satisfy respective antenna characteristics of the VHF band and the UHF band simultaneously.
For example, while it is possible to include both a receiving antenna for the VHF band and a receiving antenna for the UHF band, the inclusion of both a receiving antenna for the VHF band and a receiving antenna for the UHF band may not solve problems of space saving and size reduction. While each receiving antenna may be made detachable to allow a user to change and use the receiving antennas as appropriate, changing the receiving antennas and carrying the receiving antennas are troublesome. In addition, a number of parts are required by a matching circuit after switching a plurality of antennas or a single antenna, thus inviting an increase in mounting area and an increase in cost.
Further, a portable telephone has been proposed which has a plurality of broadcast receiving circuits and selects a demodulated signal of higher quality, so that high reception sensitivity can be obtained in any use conditions (see Japanese Patent Laid-open No. 2006-41826, for example). In this portable telephone, a loop element is disposed in the vicinity of a hinge part of an upper casing, and is connected to a broadcast receiving circuit via a matching circuit. When an earphone cable is removed, a helical element is selected by an earphone connector. A received signal of the antenna selected by the earphone connector is input to a broadcast receiving circuit. However, this portable telephone does not perform an operation of receiving broadcast waves in a plurality of required frequency bands.